Flareless fittings have been in use for decades for conduits such as tubes and pipes. A flareless fitting is used to connect or join two tube or pipe ends or to connect a conduit end to another assembly such as a tank, a valve, a manifold and so on. The applications are as varied as the types of assemblies with which the fittings are used. One very common type of flareless fitting is a ferrule type fitting. In a ferrule type fitting, one or more ferrules are used to join or connect a conduit end to a fitting member, typically called a fitting body. The fitting body may then be joined to (or be part of) another assembly. In a ferrule type fitting, the ferrule or ferrules must establish a fluid tight seal, particularly under pressure, as well as adequate grip of the conduit and protection against vibration fatigue. High performance fittings, such as are available from Swagelok Company, Solon, Ohio, are capable of withstanding pressures many times the rated pressure of the fitting without leaking, without vibration fatigue and without conduit blow out to the point that the conduit will burst before a seal is compromised or the ferrule(s) can lose their grip on the conduit.
Ferrule style fittings have an advantage over other end connections in that they do not rely on any special preparation of the tube or pipe end, other than low cost squaring and deburring. This is because the ferrules create the seals and tube grip.
Flareless fittings that use ferrules are commonly used in sophisticated chemical processing apparatus because of their high reliability. For example, in the semiconductor industry, such fittings assure containment of expensive or toxic chemicals. Typically, these applications are high purity and therefore, rely on conduits made of stainless steel or other low corrosion, high strength alloys.
Lower cost markets, such as the automotive industry, have their own performance requirements for fluid connections. Most notably, automotive assembly requires simpler assembly procedures. The automotive industry has resisted using ferrule type fittings not only for cost reasons, but also for assembly needs. Ferrules are fairly small annular members that can be dropped or lost in low cost, high throughput facilities. Typical ferrule type fittings are also assembled by what is commonly known as pull-up by turns. Two threaded components, such as a nut and body, enclose the conduit end and one or more ferrules. The assembly is first tightened to a finger tight condition and then a prescribed number of turns, such as one and a quarter or one and a half turns, are used to pull-up the fitting to its final assembled condition. The number of turns is carefully prescribed to prevent over torque or inadequate pull-up. The automotive industry on the other hand typically wants to assemble parts by torque. This allows a simple torque wrench or tool to be used to make the final assembly with the assurance that the fitting has been properly assembled.
The next generation of motor vehicles, especially for passenger use, will likely be in the area of alternative fuels such as high pressure hydrogen. The pressure ratings that will be required significantly exceed current rated pressure for ferrule type, flareless fittings (although not the pressure capability of high performance ferrule type, flareless fittings.) For example, it is expected that gaseous hydrogen will need to be contained at a pressure of 15,000 psi (15 ksi). Current flareless tube fittings are rated below 10 ksi.
For current high pressure flareless fittings, the material used for the fluid components of the fitting, in at least some applications, must be hardened. To this end, the block of material from which the fitting body is made may be a material hardened by heat treating or, in the example of an austenite stainless steel, may be manufactured from work hardened bar stock. For a straight union fitting, manufacturing the fitting from work hardened bar stock is suitable. More complex configurations, such as elbows and tee shapes, however, require substantially larger blocks of material. For these types of fittings, a substantial amount of material must be removed, which can drive up manufacturing costs.
Some threaded tube fittings, on the other hand are made from compact forgings which save material and cost. The use of forged fitting components is well accepted for low pressure applications. The forging process, however, can cause material annealing which eliminates any prior work hardening. Thus, forged components are typically unsuitable for high pressure applications.